92 AN INTRODUCTION TO ZOOLOGY 



skull in front of the cranium. There are two of these sacs side by 

 side, but completely separated from one another in the middle line 

 by the nasal septum. Each consists of a main chamber communi- 

 cating with the outside by means of the external nares, and with 

 the inside by the internal nares, which open on the roof of the 

 buccal cavity just in front of the vomers. The floor of the main 

 cavity is occupied by a well-marked swelling and two smaller cham- 

 bers, a lateral and a dorsal, lead off from it. The whole of the inside 

 of the three chambers is lined by a sensory epithelium. Transverse 

 section shows this epithelium to be simple and columnar, and to 

 contain two varieties of cells. The most important are the long 

 narrow sensory cells which are separated and kept in position by 

 more numerous columnar cells. At the free end of the olfactory 

 cell is situated a small tuft of very delicate sensory hairs, and its 

 inner end is continued on as a fine fibre. These fine fibres unite 

 together in bundles to form small nerves, which pass into the olfactory 

 lobes. Thus these cells are to be regarded as peripherally situated 

 neurons whose axons run into the brain and whose dendrons are 

 represented functionally by the small hairs, inasmuch as they 

 receive the impulse and convey it to the cell. An epithelium built 

 up in this way of neurons and supporting cells is sometimes termed 

 a nervous epithelium. Here and there the epithelium dips down to 

 form simple saccular glands, whose business it is to secrete the mucous 

 that keeps the whole interior moist. 



The main function of this organ is olfactory, that is, it is con- 

 cerned with the perception of smells of various sorts. Owing to the 

 fact that it opens internally as well as externally, it is also used for 

 respiration ; to aid in the carrying out of this latter function the 

 external nares are provided with valves that open and close, regula- 

 ting the intake of air in a way that has already been described. 



The organ of sight is the eye, situated in the orbit at the 

 side of the cranium. It is a more or less spherical structure kept in 

 position by means of its nerve and a series of muscles, which also 

 enable it to be turned in different directions at the will of the animal. 

 When removed from the skull, it will be seen that the eyeball, as it is 

 termed, is tough and more flattened on the outer than on the inner 

 side and possesses a sort of stalk, the optic nerve. The inner side 

 also is opaque, whilst the outer, exposed between the eyelids, is 

 transparent, allowing certain internal parts, the iris and the pupil, 

 to be seen through it. If cut in half, it will be found to be a hollow 

 structure with a wall composed of three layers. The external protect- 

 ing and supporting layer is termed the sclerotic, and is composed 

 of a tough cartilage which in its transparent outer portion is known 

 as the cornea. The sclerotic encloses the whole eyeball as a sort of 



